Viral infection is one of big sources of stress for plants. It is not uncommon that damages by viral diseases influence and move the habitat of crops and a variety is renewed. However, there is no effective drug which directly act upon viruses at present, so that the control of such viruses is dependent solely upon indirect means. Although it is one of important purposes of breeding to give viral resistance to a crop, there are many cases where any resistant genetic resource cannot be found in wild species or relative species which can be bred with the crop. In such a case, a virus resistant variety could not have bred according to the conventional breeding techniques such as cross-breeding and the like.
As a method for covering up such faults of the conventional breeding techniques, methods for giving virus resistance to a plant by means of genetic recombination technology have been developed recently. When the genetic recombination technology is available, not only the gene transfer which goes over the wall of the above-mentioned conventional cross-breeding becomes possible but also it is possible to introduce virus resistance directly into an existing variety which is agronomically important.
As regard as giving virus resistance to a plant by means of genetic recombination technology, methods for making virus-derived genes (a gene encoding the coat protein of a virus, cDNA of a satellite RNA) express in a plant have been reported [e.g., HortScience, vol. 25, p. 508 (1990)]. However, these methods involve the following disad-vantages.
1) A plant in which a coat protein gene of a certain virus is expressed shows resistance to the infection of the parent virus but does not show resistance to viruses of other species at all. The resistance to a virus obtained by the expression of its coat protein gene is specific to the concerned virus. The coat protein of a virus differs with that of different virus, so that, in order to give resistance to plural viruses, it is required to transduce all the coat protein genes of the plural viruses into a plant. Actually, there is a report that coat protein genes of both a potato virus X and a potato virus Y were integrated into a potato and expressed therein to produce a plant having resistance to the two viruses [Bio/technology, vol. 8, p. 750 (1990)]. However,it is obvious that such a method as above is not realistic because it requires a great deal of labor to produce a plant having resistance simultaneously to 4 or more species of viruses.
In the case, also, of a method for making a satellite RNA express, the resistance to a virus to be obtained is specific to the concerned virus. In addition, this method is lack of general application because it is inapplicable if a pathogenic virus carries no satellite RNA.
2) The method for making a coat protein gene express is utilizable only after a pathogenic virus is isolated, the genetic structure of the virus is elucidated and a coat protein gene of the virus is identified. Thus, it is inapplicable to unknown viruses.
3) A viral gene generally has high probability of causing mutation. For example, to a virus-resistant variety produced by the conventional techniques such as cross-breeding and the like, the development of a viral strain overcoming this resistance (an overcomer) is often observed in the natural world. Also to a virus-resistant plant obtained according to genetic recombination technology, there is a possibility of appearance of such an overcomer.